1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copying machine, or a facsimile, which performs image formation by forming a dot latent image, that is a dotted electrostatic latent image, on a surface of a latent image carrier, by developing the dot latent image into a toner image, and finally by transferring the toner image onto a recording medium.
2. Description of the Related Art
In an electrophotographic digital image forming apparatus, a dot latent image is formed on a surface of a latent image carrier, and a toner image is formed, for example, by attaching toner to an electrostatic latent image portion that is an irradiated portion for negative development, or to a non-electrostatic latent image portion that is a non-irradiated portion for positive development. In the following description, negative development is described as an example. An amount of toner adhering to the electrostatic latent image portion of the latent image carrier by the development is determined on the basis of the area of an electrostatic latent image, a development potential (a potential difference between the surface of the developer carrier and the electrostatic latent image on the latent image carrier), a charge amount of toner, and the like. Moreover, the gradation or image density in an output image is realized by controlling the number of dot latent images in a basic dot matrix (for example, 9 by 9 dots) (hereinafter, this control method is referred to as “area gradation control”), by controlling the adhesion amount of toner adhering to one dot latent image (hereinafter, this control method is referred to as “density gradation control”), or by performing a combination of these control methods. Specifically, a high density portion can be realized in the output image by increasing the number of dot latent images in each basic dot matrix for the area gradation control or by increasing the exposure time for the density gradation control. Conversely, a low density portion can be realized in the output image by decreasing the number of dot latent images in each basic dot matrix for the area gradation control or by decreasing the exposure time for the density gradation control.
In recent years, an optical writing time per dot has been decreased with an improvement in writing density (dot latent image density) of a dot latent image to realize a higher image quality and with improvement in image forming speed. Accordingly, in the case of employing density gradation control in which the gradation of an image is expressed by changing the optical writing time per dot, it is difficult to increase the resolution, and the number of gradation levels per dot controllable by density gradation control decreases. For example, in a high-speed image forming apparatus that forms a high-density dot latent image as high as 1200 dpi to 4800 dpi, the number of gradation levels per dot that can be realized by the density gradation control is about 4 gradation levels. Thus, it is desirable to adopt area gradation control in reproducing multi-level gradation.
On the other hand, in many image forming apparatuses, each of various kinds of density adjusting control schemes is executed at predetermined timing so as to maintain image quality (see Japanese Patent Application Laid-open No. 07-253694 and Japanese Patent Application Laid-open No. 10-013675). Specifically, for example, a patch pattern for adjusting an image density formed by a plurality of latent image patches (electrostatic latent images) that have mutually different image densities is written onto the latent image carrier, and the potentials of the respective latent image patches of the patch pattern are detected. Thereafter, the patch pattern is developed, and the adhesion amounts of toner adhering to the respective patches (toner patches) of the patch pattern are detected after the development. Then, based on the relation between the development potential obtained from the detected latent image potential and the toner adhesion amount, a predetermined value of a density index (such as a development potential for obtaining a predetermined toner adhesion amount corresponding to a reference image density (for example, a target density of a solid image)) is calculated. Then, various image forming conditions are adjusted based on the value of the density index, and control is performed so as to stabilize the image density.
When performing the density adjusting control, it is important to detect the relation between the development potential and the toner adhesion amount with high accuracy. Moreover, in order to obtain the relation with high accuracy, it is desirable to form a multi-gradation patch pattern in which a number of latent image patches of different densities over a wide density range are dispersed. However, with a decrease in the time for performing the density adjusting control, there is a limitation in the number of latent image patches that can be formed. Thus, it is required to detect the above relation with high accuracy using a multi-gradation patch pattern including as few latent image patches as possible. In order to satisfy this requirement, it is desirable to provide a multi-gradation patch pattern in which as few latent image patches as possible are dispersed in as wide a density range as possible.
However, in the image forming apparatus of the related art, the adhesion amount of toner adhering to a latent image patch of a low density portion (highlighted portion) is larger than an intended toner adhesion amount. Thus, in a case where the relation between the development potential and the toner adhesion amount is detected using the toner adhesion amount of a low-density latent image patch, the detection accuracy of the relation decreases. Moreover, when the relation between the development potential and the toner adhesion amount is detected without using the toner adhesion amount of a low-density latent image patch, the density distribution range of the patches used for detecting the relation becomes narrow. Thus, it is difficult to detect the relation with high accuracy.
FIG. 65 is a graph plotting a number of relations between the development potential and the toner adhesion amount detected in an image forming apparatus of the related art.
The relation between the development potential and the toner adhesion amount is linear and the relation can be identified by a slope and an intercept of a straight line obtained by a linear approximation of the plotted points. The approximated straight line illustrated in FIG. 65 is obtained with respect to a plurality of latent image patches in the high density portion. As illustrated in FIG. 65, all plotted points of the latent image patches in the high density portion are in the vicinity of the approximated straight line, and it can be said that the accuracy of the approximated straight line is high. On the other hand, in viewing the plotted points of the latent image patches in the low density portion, the plotted points deviate greatly from the approximated straight line toward the large toner adhesion amount side. From the above, it can be understood that the adhesion amount of the toner adhering to the latent image patches in the low density portion is larger than the intended toner adhesion amount.
In the example of the graph illustrated in FIG. 65, the number of latent image patches in the high density portion is increased and thus an approximated straight line with high accuracy can be obtained by using only the latent image patches in the high density portion. However, as described above, with a decrease in time for the density adjusting control in recent years, the number of latent image patches that can be formed is limited. Thus, it is difficult to obtain a straight line with high-accuracy of approximation using fewer latent image patches within a narrow density range only in the high density portion.
FIG. 66 is a view illustrating an example of creating a low-density latent image patch through density gradation control.
In the low-density latent image patch according to density gradation control illustrated in FIG. 66, all dots of a basic dot matrix (9 by 9 dots) are irradiated with light, and thus the potential of the entire basic dot matrix decreases uniformly in accordance with density. Moreover, an amount of toner that adheres to the low-density latent image patch is determined such that a total charge amount of the entire adhering toner is equal to the difference (development potential) between a developing bias Vb and a total latent image potential of the entire basic dot matrix. An image of the adhesion amount of toner adhering to the low-density latent image patch is illustrated in a lower part of FIG. 66. In this case, the toner adhesion amount of the low-density latent image patch becomes nearly a target amount, and a target image density is obtained.
FIG. 67 is an explanatory diagram illustrating an example in which a latent image patch, that has a latent image potential, to be detected by an electrometer, with the same level as the latent image potential of the latent image patch illustrated in FIG. 66, is formed by the area gradation control in an image forming apparatus of the related art.
In the low-density latent image patch formed by area gradation control illustrated in FIG. 67, dot latent images are written in areas, each corresponding to 3 by 3 dots, at the top-left corner and the bottom-right corner within a basic dot matrix (9 by 9 dots), so that the dot latent images are concentrated in the top-left corner and the bottom-right corner. Because a spot diameter of a beam irradiated to write one latent image is generally larger than a size of one dot latent image, light is also irradiated to adjacent dots when one latent image is written. Therefore, when a concentrated dot latent image portion is present as in the case of the low-density latent image patch illustrated in FIG. 67, a dot latent image (in particular, the dot latent image located at the center of 3 by 3 dots) is irradiated with a writing beam repeatedly. Thus, it has a latent image potential greatly less than the intended potential, and the development potential becomes much larger than the intended potential. However, when the potential of such a low-density latent image patch is detected with a general electrometer, the detection result produces a value similar to a value obtained by taking an average of the greatly decreased potential and the potentials over the entire basic dot matrix. That is, the detection result produces a similar value as that of the low-density latent image patch of the density gradation control illustrated in FIG. 66. However, the toner adhesion amount of the image adhering to a small portion which has a greatly decreased potential becomes like the one illustrated in the lower part of FIG. 67. The toner adhesion amount in that portion becomes large as compared to the toner adhesion amount in the low-density latent image patch of the density gradation control illustrated in FIG. 66. A problem associated with an increase of the toner adhesion amount is more prominent in the lower density portion where the ratio of the number of dot latent images to the total number of dots in the basic dot matrix is low.
The present invention has been made in view of the above circumstances, i.e., there is a need to provide an image forming apparatus capable of performing, with high accuracy, density adjustment using a multi-gradation patch pattern with fewer patches.